This invention relates to methods and apparatus for forming a uniform layer of material for use in connection with manufacturing a substrate assembly during semiconductor processing, and also the layer itself. The invention also relates to a method of planarizing a semiconductor wafer.
As used herein, xe2x80x9csubstratexe2x80x9d refers to the lowest layer of semiconductor material in a semiconductor wafer, and xe2x80x9csubstrate assemblyxe2x80x9d refers to a substrate having at least one additional layer with structures formed thereon.
xe2x80x9cSemiconductor flatxe2x80x9d refers to a surface of the substrate assembly having a precision flat surface within desired tolerances. A significant aspect of semiconductor processing is planarization, i.e., ensuring that the semiconductor flat and other layers are planar within a predetermined specification.
Production methods for semiconductors are known. A particular class of methods involves: etching or otherwise forming desired channels or trenches in a substrate assembly surface, applying a dielectric epoxy layer which fills the trenches over the substrate assembly surface, using an apparatus to press the substrate assembly having the epoxy layer to achieve desired surface characteristics (e.g., flatness) on the epoxy layer, and then removing the pressed substrate assembly from the apparatus for further processing. The epoxy may be of a type which is cured with ultraviolet radiation.
Removing the pressed substrate assembly from the apparatus is difficult, however, because the epoxy begins bonding with the pressing surface. Therefore, according to some methods, the epoxy layer is first covered with a layer of a cover material before the pressing takes place. The cover material is selected to allow easy removal/release of the pressed substrate from the apparatus.
In addition, the cover or release member must be transparent to the ultraviolet radiation if an epoxy of the type cured by ultraviolet radiation is used to cure the epoxy layer beneath the cover material. It has been previously determined that fluorinated ethylene-propylene (FEP) can be used as the cover material. Some types of FEP are transparent to ultraviolet radiation, and thus do not affect the epoxy curing by ultraviolet radiation passing through the cover.
The cover material is placed over the epoxy layer before the substrate assembly is pressed, and thus the cover material surface characteristics are transferred to the substrate assembly surface. If the cover material is a uniform layer, which is defined as a layer having parallel major (top and bottom) surfaces that are planar, within predetermined tolerances, the pressing action applied through the cover material will be uniformly transferred to the epoxy layer as desired. As one result, if the cover material is a uniform layer, the substrate assembly surface can be formed to the same flatness as the pressing surface.
In practice, achieving a sufficiently uniform layer of a cover material such as of FEP has not been achieved utilizing known techniques. Because of the nature of FEP material and the desired thickness of a typical cover (about 0.020 in. thick), the dimensions of a FEP cover are difficult to control. For example, in one approach where ultraviolet transmissive FEP has been heated to a temperature below its melting point and pressed between two optical flats during pressing, the major surfaces of the resulting FEP layer end up significantly skewed or out of parallel from one another. As used herein, optical flats are defined as precision pressing surfaces, e.g., surfaces that are flat to within one quarter of a wavelength of light.
The temperature range for processing the FEP is very narrow. An acceptable temperature is slightly below the melting glass flow transition point, which allows the FEP material to acquire the surface smoothness characteristics of the optical flats. Since high pressures are required to make the FEP surface conform to the optical flats surfaces, at temperatures below the glass transition point (i.e., in the plastic state), maintaining the material at a consistent thickness is very difficult. This difficulty is due to the uncontrolled movement of FEP material from the higher pressure zones to the lower pressure zones at the perimeter of the pressing mechanism. Consequently, the thickness of the layer is no longer satisfactorily uniform.
When used as a cover layer, this non-uniformity in thickness caused variations in the thickness of the epoxy layer. Consequently, during subsequent semiconductor wafer processing, involving etching through the epoxy layer, undesirable non-uniform etching would occur because thinner portions of the epoxy layer would be etched through first. For example, FEP sheets exhibiting these problems had major surfaces which were flat to within about 30-35 angstroms, but which were only parallel to one another within xc2x10.010 in., have been obtained using known processes.
Accordingly, it would be desirable to provide a method and apparatus by which FEP and other materials used as cover layers on a substrate assembly could be produced within desired uniform layer specifications.
Wafer planarization is enhanced utilizing a layer of material having a substantially uniform thickness and substantially parallel first and second major surfaces. The layer is used in producing a flat on or planarizing a substrate assembly.
In one embodiment, an apparatus having a substantially uniform thickness and substantially parallel first and second major surfaces includes a pair of pressing elements and a stop. The layer of material formed by the apparatus used in producing a flat on semiconductors. Each of the pair of pressing elements has a flat pressing surface. The pressing surfaces are opposed to one another and operable to compress a quantity of the material therebetween. The stop is positioned at least partially between the pressing surfaces and has a thickness substantially equal to the desired uniform thickness of the layer. The stop is positioned to establish a spacing between the flat pressing surfaces that is substantially equal to the thickness of the stop and thereby to the desired uniform thickness of the layer when the pressing elements engage the stop. As a result, engagement of the stop by the pressing surfaces during pressing of the material forms a layer of the material of substantially uniform thickness with substantially parallel major surfaces formed by the flat pressing surfaces.
The apparatus can also include a heater that heats the material to a temperature where it flows without melting. Further, the apparatus can include a compression force applicator to move one or both of the pressing surfaces. The compression force applicator can include a plurality of biasing elements.
The pressing surfaces can be optical flats. The shim can have a plurality of projections extending inwardly from the border portion with overflow material recesses positioned between the projections. The projections can be of a triangular shape.
In a specific example, the first and second major surfaces of the layer are each within 100 angstroms of being flat. Preferably, in this example, the first and major second surfaces of the layer are at least within 0.000005 in. of being parallel to one another. In this example, a stop portion of the shim is about 0.020 in. thick. The cover layer may also be transparent to ultraviolet radiation.
According to an exemplary method, a layer is formed by heating material and pressing the material between first and second flat pressing surfaces. A stop is disposed between the first and second pressing surfaces to limit the extent to which the first and second pressing surfaces approach one another during pressing to thereby form a layer of substantially uniform thickness having first and second major surfaces with the first and major second surfaces being formed by the flat pressing surfaces. Thereafter, one of the first and major second surfaces of the formed layer may be applied to a flat surface of a substrate assembly. In this approach, the heating step may include heating the material until the material transitions to a plastic state without melting.
The formed layer may be applied, for example, over an epoxy layer of a substrate assembly. The assembly may then be pressed by precision optical flats with the flatness of the optical flats being transferred to the epoxy layer through the formed layer. The formed layer in this case prevents the epoxy layer from adhering to the pressing apparatus.